1. Field of the Invention
The present invention relates to a gallium nitride type semiconductor laser device for use in a light source of an optical disk system.
2. Description of the Related Art
A gallium nitride type semiconductor (e.g., GaInAlN) is used as a semiconductor material for a semiconductor laser device (LD) having an emission wavelength in a wavelength range from ultraviolet to green. For example, MRS Internet J. Nitride Semicond. Res., vol.2, no.5 (1997) describes a semiconductor laser device using such a gallium nitride type semiconductor, as illustrated in a cross-sectional view in FIG. 11.
Referring to FIG. 11, the semiconductor laser device has, on a sapphire substrate 201, a GaN buffer layer 202, an n-GaN contact layer 203, an n-In0.05Ga0.95N layer 204, an n-Al0.08Ga0.92N cladding layer 205, an n-GaN guide layer 206, a multiquantum well structure active layer 207 including In0.15Ga0.85N quantum well layers and In0.02Ga0.98N barrier layers, a p-Al0.2Ga0.8N layer 208, a p-GaN guide layer 209, a p-Al0.08Ga0.92N cladding layer 210, a p-GaN contact layer 211, a p-side electrode 212 and an n-side electrode 213. The multiquantum well structure active layer 207 includes seven layers in total, i.e., four In0.05Ga0.85N quantum well layers each having a thickness of about 3.5 nm and three In0.02Ga0.98N barrier layers each having a thickness of about 7 nm. In the multi-quantum well structure active layer 207, the quantum well layers and the barrier layers alternate with each other.
In this conventional example, the p-Al0.08Ga0.92N cladding layer 210 and the p-GaN contact layer 211 are formed in a ridge stripe pattern so as to constrict an injected current. The width of the stripe pattern is about 4 μm.
Japanese Laid-open Publication No. 9-232680 describes a semiconductor laser device similarly using a gallium nitride type semiconductor, which also includes a ridge stripe structure having a stripe width of about 5 μm to about 10 μm for constricting an injected current.
When employing a semiconductor laser device using a gallium nitride type semiconductor as a light source of an optical disk system, in order to prevent a read error from occurring due to noise during a data read operation, a self-pulsation type semiconductor laser is employed in which an optical output is modulated for a constant current injected. Such a semiconductor laser device is described in Japanese Laid-open Publication No. 9-191160, for example.
FIG. 12 is a cross-sectional view illustrating such a semiconductor laser device. Referring to FIG. 12, the semiconductor laser device includes an n-SiC substrate 221, an n-AlN buffer layer 222, an n-Al0.15Ga0.85N cladding layer 223, an In0.15Ga0.85N active layer 224 having a thickness of about 50 nm, a p-Al0.15Ga0.85N first p-type cladding layer 225, a p-In0.25Ga0.75N saturable absorbing layer 226, an n-Al0.25Ga0.75N current blocking layer 227, a p-Al0.15Ga0.85N second p-type cladding layer 228, a p-GaN cap layer 229, a p-GaN contact layer 230, a p-side electrode 231 and an n-side electrode 232.
In this conventional example, a portion of light generated by the active layer 224 is absorbed by the saturable absorbing layer 226, thereby causing an absorption coefficient of the saturable absorbing layer 226 to change. Accordingly, an intensity of light emission by a laser oscillation from the active layer 224 is changed periodically. As a result, coherence of the emitted light from the laser is reduced. This conventional example also includes a ridge stripe structure having a stripe width of about 2 μm for constricting an injected current.
When employing such a semiconductor laser device with reduced coherence as a light source of an optical disk system, even if light reflected by the disk returns to the semiconductor laser, the emitted light from the laser does not interfere with the reflected return light, thereby suppressing generation of noise and thus preventing a data read error from occurring.
However, the conventional laser device using a gallium nitride type semiconductor has the following problems.
First, in the self-pulsation type semiconductor laser device having the saturable absorbing layer, light generated by the active layer is absorbed by the saturable absorbing layer, thereby increasing the loss of light within the laser cavity. As a result, the oscillation threshold current of the semiconductor laser device increases, and the emission efficiency is reduced. Moreover, in this conventional self-pulsation type semiconductor laser device, since the saturable absorbing layer is added only to one of the cladding layers interposing the active layer therebetween or only to one of the guide layers interposing the active layer therebetween, the far field pattern of the emitted light from the laser is asymmetric, whereby the focused spot size cannot be made sufficiently small when focusing the emitted light with a lens.
The conventional laser device using a gallium nitride type semiconductor to which the saturable absorbing layer is not added does not have such problems (e.g., the increased oscillation threshold current, the reduced emission efficiency, and incapability to have a small focused spot size) as those seen in the conventional self-pulsation type semiconductor laser device. However, when this semiconductor laser device is used as a light source of an optical disk system, noise occurs due to the return light from the disk, thereby causing a read error during a data read operation. Therefore, the conventional laser device using a gallium nitride type semiconductor to which the saturable absorbing layer is not added is not suitable for a light source of an optical disk system.